FACTSHEET 14/15 Soft Fruit

Projects SF 80, SF 90 and Defra Horticulture LINK Project HL01107 Scott Raffle, AHDB Horticulture, Jude Bennison, ADAS, Jean Fitzgerald, East Malling Research and Clare Sampson, Keele University. Western flower control in strawberry

Western flower thrips (WFT) cause significant financial losses for strawberry growers in the United Kingdom. This factsheet provides information on the pest, the damage it causes to strawberries and the results of AHDB funded research in Projects SF 80, SF 90, SF 120 and a study of control in commercial strawberry production sites, which have led to a series of control guidelines.

1. WFT feeding in strawberry flower 2. Damage to strawberry caused by WFT feeding

Action points

Most successful control of WFT in commercial • Phytoseiulus persimilis is used as the main control method strawberry production has been found where: for two spotted spider mite. • Strawberry crops are only grown for one season. • Crop protection programmes that are harmful to predators are avoided. • Well-managed regular predator release strategies are used in all crops from either before flowering or from the • Advice on biological control programmes is sought from an first flowers, using , combined with adviser who is experienced in using predators. one or more of: Stratiolaelaps scimitus, species • Product compatibility tables are consulted to check if (later in the season when temperatures are high enough proposed crop protection products are likely to be harmful for establishment) or mass trapping with blue sticky to the predators being introduced. roller traps. Introduction Recognition and biology

Western flower thrips – WFT ( occidentalis) first was causing serious damage to developing flowers and fruits Adults became a pest of protected ornamental and edible plants and this prompted AHDB to commission a series of research in the UK in the 1980s and considerable research has been projects on WFT control in strawberry, running from 2006 Like other thrips species, WFT adults are small slim , done since then on management in protected crops. It wasn’t until 2015 and a further study in 2014, comparing poor and with narrow wings fringed with hairs. Female WFT are slightly until 2002 and later that the pest was found to be damaging successful control on commercial sites. This culminated in larger (1.5–2mm long) and darker than the males. The females protected and outdoor strawberry crops, the first outbreaks a set of management and control guidelines for commercial (Figure 4) usually have a yellowish head and front half of the being found in the midlands of England. Feeding by the pest strawberry growers that are summarised in this factsheet. body and a brownish back half. The males are yellowish all over. Adults can live for several weeks. The temperature threshold Thrips species for WFT flight is 20°C but they can be active and egg laying in flowers at temperatures below this. Confirmation of species There are many different species of thrips native to the Fruit bronzing similar to that caused by WFT has been seen present in the crop needs to be done under a microscope. UK. Those that can commonly occur in strawberry flowers where onion thrips or rose thrips are present, with rose thrips include the onion thrips (), the rose thrips (Thrips (Figure 3) in particular being found in high numbers in strawberry Eggs fuscipennis) and Thrips major (no common name). Often these flowers in recent years. Cereal thrips (eg Limothrips cerealium) 4. WFT adult female occur in species mixtures. WFT is not a native species of the can reach very high numbers in warm thundery weather, at UK, having been first introduced to the UK on infested plant harvest time and when cereal crops/hay/grass verges are cut WFT eggs are not visible as the females lay them directly into plant material in 1986. It thrives in warmer temperatures and with the during the summer months and may occasionally be found tissue. Time to egg hatch is about five days at 20°C. increase in strawberry production under protected structures, in strawberry flowers, but they do not breed on strawberry or the pest is now widespread in all areas of the UK including cause significant damage. Scotland. Larvae

Larvae (Figure 5) hatch from the eggs after a few days, depending on temperature. There are two larval stages, neither of which have wings. First stage larvae are less than 1mm long and colourless or white. They feed for two or three days before developing into the second stage larvae, which are about 1mm long and yellow. The second larval stage lasts for about 11 days at 20°C and five days at 25°C, but develops more slowly at lower temperatures. When fully grown, most of the late second stage larvae drop from the plants to pupate in the soil, growing media or substrate. However, a small proportion may pupate on the plants, in sheltered places such as behind the calyx on fruit or hidden in flowers.

5. WFT larva

Pupae

There are two pupal stages, the prepupa and the pupa (Figure 6), neither of which feed. Both these stages are yellowish in colour and about 2mm long. The prepupa has short immature wings and short, forward-pointing antennae. The prepupal stage lasts only for a day or two then develops into the pupa, which has longer immature wings and longer antennae that fold backwards over the body. The pupal stage lasts for about five days at 20°C and three days at 25°C. When pupation is complete, the adult thrips emerges from the pupal case.

6. WFT pupa

3. Rose thrips are commonly found in strawberry flowers and should not be confused with WFT

2 3 FACTSHEET 14/15 Western flower thrips control in strawberry Survival and development Crop damage

Research in Project SF 80 confirmed that WFT tends to be a Damage is often first seen on the flower petals as characteristic brown markings around the seeds and under the calyx and have greater problem in fields with a history of WFT and where plants or brown markings (Figure 9), but WFT adults and larvae prefer to a dull, brown or bronzed appearance when ripe, rendering the beds are kept from one year to the next. It was found that adults feed on the flower receptacle, and significant damage may have fruits unmarketable (Figure 10). Yield loss of 15–20% is typical in emerged in March from the soil in planting holes directly under the occurred to an individual flower before the petals become marked. strawberry crops where WFT control has broken down. Where strawberry plants, the soil or substrate close to the plants within When numbers of WFT are high, damage occurs rapidly and the high populations develop, on everbearers for example, even more the raised beds or grow bags, the straw in the alleys and the soil whole flower may gain a brown unsightly appearance. Feeding severe crop losses can occur, leading to crop write-offs in some under the straw. Project SF 120 also found WFT overwintering as continues on the surface of developing fruits, which also develop cases. adults in senescent or dead strawberry flowers and weeds such as chickweed, groundsel (Figure 7) and dandelion and the first larvae were found in groundsel flowers in late March, indicating that females were laying eggs in early March. WFT are often found on the edges of the crop and especially in weedy field margins. Second year crops can start the season with significantly more WFT that first year crops. Western flower thrips eggs, larvae and pupae can develop slowly below 10°C, but most information on development rates has been obtained at 10–35°C. WFT can survive in a quiescent state at lower temperatures over the winter. At typical summer glasshouse fluctuating temperatures, WFT can develop from egg to adult in only 11 days. The pest can therefore breed very quickly during the summer and can go through many generations per year, particularly in heated structures (Figure 8) and on everbearers, which have a long season with several flower flushes. Once the pest is established, the highest numbers tend to be concentrated where temperatures are higher, and in the case of field grown 8. Many generations of WFT can occur in heated glasshouse structures crops, in the mid to top areas of sloping fields. 9. Brown markings on flower petals caused by WFT feeding 10. Strawberry fruits damaged by WFT

Monitoring for thrips in strawberry

A sex aggregation pheromone specific to WFT was developed by scientists at Keele University and is available from Syngenta Bioline. The pheromone is produced by WFT males and attracts both male and female WFT for mating. The pheromone is supplied commercially, adsorbed onto small rubber lures that are stuck onto the lower section of blue sticky traps (available from most Integrated Pest Management – IPM – suppliers) to help monitor for WFT. A different thrips attractant for various thrips species including WFT is available from Koppert. As this attractant is not selective, it will not help to distinguish thrips species. If used on sticky traps it will also attract natural enemies such as Orius, so possibly reducing numbers of predators in the crop. 11. Blue sticky trap used for monitoring before flowering Before the crop starts flowering, early emerging WFT adults that have overwintered in the crop can be found on flowering weeds. At this time, blue sticky traps together with the pheromone lure are very effective at catching WFT. Each trap can catch several hundred adults per week at times in Spanish tunnels in the UK, depending on the numbers of WFT that have overwintered in the crop. In strawberry, the best position for traps is to mount them onto a post (a cheap bamboo cane is sufficient), held in place with a rubber band with the bottom of the trap (landscape orientation) about 10cm (one hand width) above the top of the crop, orientated to face south so that it catches more light (Figure 11). Once the strawberry flowers start to open, counts of thrips in flowers have been found to give a better correlation with damage than pheromone trap catches. This is because thrips flight onto sticky traps varies with the daily temperature and wind strength, so does not reflect the population density as well as counts of thrips in flowers. 12. Use of a hand lens is essential when monitoring for thrips 7. WFT can overwinter in certain weed species such as groundsel

4 5 FACTSHEET 14/15 Western flower thrips control in strawberry The first strawberry flowers in tunnel crops are usually seen in are seen as brown rasping under the calyx. Adult thrips should rely upon traps on their own to control WFT, but only as part of an March. They attract adult thrips, and the females immediately start be easily found at this stage. Bronzing damage to strawberry fruit integrated control programme with predators. The traps are most to lay eggs. The thrips concentrate in the yellow stamens and increases with increasing numbers of adult thrips per flower. Green useful at sites with a history of poor WFT control and in higher risk anthers as they feed on the pollen. When monitoring for thrips in and white berries of all sizes will be bronzed, remaining so until crops (eg second-year crops). They were more effective from July strawberry flowers, the selection of flower age and position affects ripe. Damage may only affect parts of a berry, but in severe cases to September when thrips more frequently than in April to June population estimates. For monitoring thrips adults, select flowers the whole berry can be affected. Berry bronzing can be seen as when it is cooler. of medium age (all petals present, pollen shed) from the top of the early as April under tunnels, as green berries develop, but damage The traps remain sticky for two months, but after they are first plant, as young (petals fresh, pollen not shed) or senescent (petals levels to everbearer fruit are often highest in July and August, when rolled out, there is a lag before they have any effect (as it takes time dropping) flowers will result in an underestimation. For monitoring WFT populations have increased. for thrips numbers to build up in the crop), so it is recommended thrips larvae, select senescent flowers (petals dropping). Research in Project SF 120 showed that both the adult and larval to put them out about three to four weeks before damage is Look in the flower centres using a x10 hand lens (Figure 12 – page stages of WFT can cause damage to all stages of flowers and expected. It should be noted that the aggregation pheromone is a 5). The yellow parts (carpel) host the thrips, although the adults fruit, with the larvae found to cause about twice as much damage precision monitoring tool and there is no approval for its use as a are commonly found crossing over the petals, into and around the as the adults. On everbearers, the worst damage occurs to fruit control agent in commercial crops at this time. sepals. developing from flowers occurring towards the end of a flower flush, when the thrips adults congregate on the few available Low numbers of adults can be missed as they weave deeply flowers, thereby increasing the pest numbers per flower. amongst the stamens. There are various ways of exposing the Biological control adults: In initial studies on biological control of WFT using the predatory • Tap the flower sharply onto a hand or preferably a piece of Damage thresholds mite Neoseiulus cucumeris in Project SF 80, it was demonstrated paper on a clipboard or in a notebook and watch adults that use of the predator could reduce the numbers of damaged and larvae run for cover. This is the best way to count Research on damage thresholds in everbearers (Project SF or bronzed fruit when compared to untreated control plots (Figure numbers of thrips per flower. Tap a minimum of 20 flowers 120), demonstrated that in the absence of predatory mites to 14). It should be noted that N. cucumeris feeds on the first larval to get a good idea of the mean numbers per flower, as control WFT, significant damage that might result in downgrading stage of WFT, but not the adults or second stage larvae. Like other numbers can vary considerably between flowers. of fruit, occurred when there were about four adult thrips per predatory mites, it seeks its prey by crawling across the foliage, • Gently blowing into the flower centre forces the adults out flower on cv. Camarillo. However, in controlled experiments, flowers and fruit. Orius laevigatus predatory bugs were also found 15. Sachet formulations of Neoseiulus cucumeris onto the petal. when the predatory mite Neoseiulus cucumeris was present in to offer some level of biological control. A number of commercial flowers, this figure was higher, with significant damage being growers have had some success with N. cucumeris, so further • Peel the petals and sepals back and look straight into the avoided when there were up to eight adult thrips per flower. No work was done in the Defra Horticulture LINK project (SF 120) to side of the carpel. detailed information on damage thresholds is available for other determine the best ways to improve efficacy. everbearer varieties. Interestingly, in experiments in commercial crops where predators Understanding how and why damage develops had been released but their establishment had been poor, during the season economic damage was observed where numbers of adult WFT were at five per flower or above. In contrast, in commercial The first signs of damage will occur in fields where growers have crops where predators had been released and established well, failed to gain control of WFT with spinosad (Tracer) or where an economic damage was not observed until numbers of adult IPM programme has not been well managed. A microscopic WFT were as high as 11 adult thrips per flower. examination by an entomologist is the only way to make an accurate identification of thrips species. Early signs of damage

Control 16. Loose formulation of Neoseiulus cucumeris in a strawberry crop Within the Defra and AHDB funded projects, research has Sticky roller traps investigated three forms of control including the use of sticky roller the experience of ADAS consultants working closely with grower traps, biological control using predatory mites and bugs and the Research was done (Project SF 120) to assess the efficacy of businesses where successful control is now regularly achieved. use of biopesticides and selective crop protection products. 14. Neoseiulus cucumeris successfully reduces WFT populations using blue sticky roller traps (30cm wide, 100m long) with and An AHDB Horticulture training video is available on the AHDB without additional WFT aggregation pheromone along the tunnel Horticulture website to guide staff on how best to introduce the To provide acceptable control of WFT, it was found that it is best legs to reduce adult WFT populations (Figure 13). The traps were predatory mite N. cucumeris to a strawberry crop. tested in addition to releases of predatory mites as part of an IPM to release N. cucumeris early in the season before thrips numbers programme. In one season, the traps on their own reduced WFT begin to increase. In commercial practice, this is normally in Further work in Project SF 120 investigated the use of Orius numbers in the crop by 61% and resulting fruit damage by 55%, mid-March in tunnel grown crops. It was shown in the trials that laevigatus (Figure 17 – page 8) to supplement the control gained giving a direct economic benefit to growers of about £2,000 per equal control was given whether the predators were released by N. cucumeris. Unlike N. cucumeris, Orius adults can fly to find ha. With the addition of the pheromone, use of traps reduced WFT in sachets (Figure 15) or in the loose formulation (Figure 16). WFT and the predator feeds on both adults and larvae, which numbers by 73% and fruit damage by 68%. However, the traps However, whichever release system is used, it is necessary to is advantageous. However, it only works well in the warmer alone (without predatory mites) were not sufficient to prevent fruit continue to release N. cucumeris regularly throughout the season. months from late May and June onwards, as it needs a minimum damage in crops with high thrips numbers. In 11 trials over three Commercial success has been achieved in tunnel everbearer of 15°C for egg-laying to occur and needs over 20°C for good seasons, the traps typically reduced thrips numbers by about crops when the predator has been released using the loose establishment. As a result, populations take time to build up so 50%, however results were variable. In some trials, WFT numbers product every two weeks from mid-March onwards, when can’t be relied on as a preventive measure. Commonly used were successfully reduced by the traps, but not the level of fruit flowering is beginning, through to mid-September. The normal rate rates of introduction for Orius are a minimum of 0.25 to one bug damage. Where WFT numbers were low, for example, when good at present for prevention of thrips is 25 per plant per application per strawberry plant, which should be repeated two weeks later. 13. Blue sticky roller traps can in some seasons, biological control programmes were used, the traps did not give although this rate is often increased to 50 per plant if numbers of Higher release rates have successfully been used in recent years help to reduce numbers of WFT a benefit. As research results were variable, growers should not thrips per flower increase. This rate of introduction is based on when temperatures have allowed good establishment.

6 7 FACTSHEET 14/15 Western flower thrips control in strawberry Biopesticides Reasons for success and failure occurring in commercial practice

Work in Project SF 120 investigated the use of several biopesticides for controlling WFT. Products tested included foliar applications of Mycotal (Lecanicillium muscarium), foliar and drench applications of Naturalis-L (Beauveria bassiana – Figure 19), soil drench application of Met52 ( brunneum – formerly ) and foliar and drench applications of a coded botanical biopesticide. Despite some favourable results using Naturalis-L in laboratory based experiments, in field experiments none of these products were effective. Further work is required to evaluate the persistence of the fungal spores on the strawberry crop and to determine the number of spores per thrips

17. Orius laevigatus, which can be used to required to cause death. supplement Neoseiulus cucumeris

18. Stratiolaelaps scimitus

In commercial practice, the use of the soil living predatory mite Stratiolaelaps scimitus (formerly Hypoaspis miles) to control thrips larvae (when they fall to the ground to pupate) and pupae, has 19. Thrips infected with Beauveria bassiana been found to help augment control by other methods (Figure 18). 2 Preventive introductions of 100mites/m are normally used. Traditional crop protection products 20. Particularly heavy losses to western flower thrips occurred in 2013 and 2014 The biocontrol costs of using this type of programme are high, but Soon after the first appearance of WFT in UK grown strawberry are still significantly less expensive than the financial losses incurred crops, the AHDB secured an Extension of Authorisation for Minor In 2013 and 2014, a significant number of UK strawberry by WFT damage to the crop, which can be >40% of lost tonnage Summary of the crucial findings Use (EAMU) for the use of Tracer (spinosad) on strawberry crops growers suffered particularly heavy losses of yield as a result of in everbearer crops. to control WFT. However, experience of using spinosad overseas failing to achieve acceptable control of WFT (Figure 20). In light WFT control was most successful where: It should be noted that products commonly used for capsid had indicated that populations of WFT can develop resistance to of the knowledge gathered both from AHDB funded research control in commercial strawberry plantations such as the pyrethroid this product quite rapidly. In anticipation of resistance developing in projects and related experience gained working with commercial • Only one-year strawberry crops were grown. lambda-cyhalothrin (Hallmark) and the organophosphate the UK, AHDB funded Project SF 90 to assess alternative control strawberry producers, AHDB Horticulture funded a study of • Well-maintained regular predator release strategies chlorpyrifos (Equity), are very harmful to, and persistent against, products. selected strawberry production sites. The aim of this study was to (Figure 21 – page 10) were employed (in first – or N. cucumeris and O. laevigatus. Horticulture LINK Project HL 0184 identify factors that may have led to the success or failure of WFT A number of approved and non-approved products were second-year crops) either from or before flowering, using (HDC Project PC/SF 276) led to the commercial development of control in tunnel grown strawberry, which could be disseminated to included but none of these consistently reduced numbers of WFT Neoseiulus cucumeris combined with: capsid traps, which contain lures using the female sex pheromone growers to improve their control. during flower development throughout the trial. Interestingly, the – Stratiolaelaps scimitus (formerly named Hypoaspis miles) of common green capsid and European tarnished plant bug. population of WFT used in the second year of this trial was shown Six growers were visited or interviewed (two in Kent, three in and Orius spp. These traps are commercially available and enhance the early to be resistant to Tracer. Staffordshire and one in East Anglia) and details were collected or detection of these pests, which could allow for more timely from six crops where WFT control was successful and six crops – Stratiolaelaps scimitus and mass trapping with blue sticky application of crop protection products. Reducing weeds within In the early stages of Project SF 120, some other coded products where it had broken down. Growers provided more general details roller traps. the crop will also help to reduce the capsid threat. were assessed for their potential at controlling WFT and compared about their site history, growing details, pest and disease control or with Tracer. In Year 2 of the project, two coded products Similar harmful products may also need to be used in the control methods, monitoring methods and decision-making processes. – Stratiolaelaps scimitus, Orius spp. and mass trapping compared favourably with Tracer, providing 80 and 81% control of spotted wing drosophila, Drosophila suzukii, (SWD). It is Treatments were reported up to the end of July 2014, which with blue sticky roller traps. respectively, compared to 69% control achieved by Tracer. These therefore vital that good early season biological control of WFT is reflected the time of the survey and included the early season, or coded products are not currently approved for use on strawberries achieved before the use of any such products are required. Failure which is the key time for establishing natural enemies that are – Mass trapping with blue sticky roller traps. in the UK. AHDB Horticulture is investigating the potential for to control WFT early in the season will create a conflict in control essential for WFT control. securing future approvals on strawberry. • Phytoseiulus persimilis was used as the main control options for both pests later in the year when SWD populations method for spider mites. start to increase. However, in the short to medium term, growers will need to rely primarily on the use of biocontrol techniques for controlling WFT. • Pesticide programmes that are harmful to predators were avoided.

8 9 FACTSHEET 14/15 Western flower thrips control in strawberry 21. Success is achieved using regular predator releases either from or before flowering

WFT control broke down where: Specific control guidance for growers • There was a large carry-over of WFT from the previous season, either from overwintered first-year crops or from At the end of the study to identify the differences between the reused, untreated growbags, resulting in damage at first successes and failures, practical guidance was produced for flowering. growers to follow to achieve successful control of WFT. This guidance is reproduced in the insert in the back cover of this • Predators were released too late. publication. • InsufficientN. cucumeris releases were made early in the season. • Crop protection products (Figure 22) that are harmful to predators were used during the time when predatory mites were being released, or there was repeated use of slightly/moderately harmful products, which prevented predator establishment and interrupted WFT control.

POCKET?

22. Control breaks down when products that are harmful to predators are applied during the predator establishment phase

10 Defra Horticulture LINK project (HL01107 – SF 120) Further information consortium details

Other useful publications Project leader: Jerry Cross, East Malling Research Industry leader: Richard Harnden, Berry Gardens Growers AHDB Grower guide. Biocontrol in soft fruit. Consortium members: AHDB Factsheet 14/09. Thrips control on protected East Malling Research ornamental crops. Natural Resources Institute AHDB Study report. Management of pesticide-resistant ADAS western flower thrips on tunnel-grown strawberry: a study Keele University of the reasons for successes and failures on commercial Warwick Horticulture Research International (HRI) production sites. AHDB Horticulture Bayer CropScience AHDB Biocontrol training video. Using Neoseiulus cucumeris Belchim Crop Protection for western flower thrips and tarsonemid mite control Berry Gardens Growers (translated into Bulgarian, Romanian, Latvian, Lithuanian, BerryWorld Polish, Slovakian). Certis UK CPM Retail East Malling Limited Useful AHDB project reports Russell IPM Syngenta Bioline SF 80 – Tunnel-grown everbearer strawberry: biology and Tesco. integrated control of western flower thrips (Project leaders: Jude Bennison, ADAS and Jean Fitzgerald, East Malling Research). Image credits SF 90 – Chemical control of western flower thrips in strawberry flowers (Project leader: Jerry Cross, East Malling The following is a list of the copyright on each of the figures Research). included in this factsheet: SF 120 – Biological, semiochemical and selective chemical Figures 1, 2, 5, 7, 9, 10, 11, 12, 13, 15, 16, 19, 20, 21, 22 management methods for insecticide resistant western © ADAS flower thrips on protected strawberry (Defra Horticulture LINK Figures 3, 4, 6, 17 © Nigel Cattlin/FLPA Images of Nature HL01107). Figure 8 © AHDB Horticulture PC/SF 276 (HL0184) – Pheromone technology for management of capsid pests to reduce pesticide use in Figure 14 © Syngenta Bioline horticultural crops (Project leader: Michelle Fountain, East Figure 18 © Certis BCP Malling Research).

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